Polycarbonate and polyester films and the like, which have conventionally been used as optical films, have a large photoelasticity coefficient, and consequently minor variations in stress can cause variations in the retardation imparted to transmitted light. Furthermore, acetate films such as triacetylacetate suffer from problems relating to heat resistance and deformation on water absorption.
Cyclic polyolefin-based resins have properties which include a high glass transition temperature due to the rigidity of the primary chain structure, a non-crystalline structure with a high light transmittance due to the presence of bulky groups on the primary chain structure, and a low birefringence due to the small anisotropy of the refractive index, and are consequently drawing considerable attention as transparent thermoplastic resins with superior heat resistance, transparency and optical characteristics.
Examples of this type of cyclic polyolefin-based resin are disclosed in Japanese Laid-open publication (kokai) No. 1-132625 (JP1-132625A), Japanese Laid-open publication (kokai) No. 1-132626 (JP1-132626A), Japanese Laid-open publication (kokai) No. 63-218726 (JP63-218726A), Japanese Laid-open publication (kokai) No. 2-133413 (JP2-133413A), Japanese Laid-open publication (kokai) No. 61-120816 (61-120816A), and Japanese Laid-open publication (kokai) No. 61-115912 (JP61-115912A).
In recent years, the utilization of the above properties in the application of cyclic polyolefin-based resins to optical materials such as optical disks, optical lenses and optical fibers and the like, as well as to fields such as sealing materials for optical semiconductor sealing has been the subject of much investigation.
Similarly, in the field of optical films, the above properties of cyclic polyolefin-based resins are capable of resolving the aforementioned problems associated with conventional resins, and as a result, films formed of cyclic polyolefin-based resins have been proposed as optical films for a variety of different film applications. Specific examples include the retardation plates formed of cyclic polyolefin-based resin films disclosed in Japanese Laid-open publication (kokai) No. 4-245202 (JP4-245202A), Japanese Laid-open publication (kokai) No. 4-36120 (JP4-36120A), Japanese Laid-open publication (kokai) No. 5-2108 (JP5-2108A) and Japanese Laid-open publication (kokai) No. 5-64865 (JP5-64865A). The application of cyclic polyolefin-based resin films to protective films for polarizing plates is disclosed in Japanese Laid-open publication (kokai) No. 5-212828 (JP5-212828A), Japanese Laid-open publication (kokai) No. 6-51117 (JP6-51117A) and Japanese Laid-open publication (kokai) No. 7-77608 (JP7-77608A). Moreover, a liquid crystal display element substrate formed of a cyclic polyolefin-based resin film is disclosed in Japanese Laid-open publication (kokai) No. 5-61026 (JP5-61026A).
In the applications described above, the fact that a cyclic polyolefin-based resin with a water absorption of no more than 0.05% can be produced with relative ease, and this low water absorption value are reported as the special characteristics of the resins, and are described as essential properties. However, if this type of low water absorption cyclic polyolefin-based resin film is used as a retardation plate or a liquid crystal display element substrate, then the film may suffer from inferior adhesion to a hard coat, an antireflective film or a transparent conductive layer, or inferior bonding with the polarizing plate and the glass respectively. In cases in which a cyclic polyolefin-based resin film is used as a protective film for a polarizing plate, then in addition to the adhesion problems outlined above, an additional problem arises in that the water of the aqueous adhesive typically used for bonding the film to the polarizer is very difficult to dry.
However, the cyclic polyolefin-based resin family includes a wide range of different structures, and not all cyclic polyolefin-based resins display a water absorption of 0.05% or less. In order to ensure a water absorption of no more than 0.05%, the cyclic polyolefin-based resin must have either a polyolefin structure formed from only carbon atoms and hydrogen atoms, or a structure which incorporates a proportion of halogen atoms.
Consequently, in order to resolve the aforementioned problems relating to low water absorption, optical films incorporating a thermoplastic norbornene-based resin with a polar group incorporated within the molecular structure have been-disclosed in Japanese Laid-open publication (kokai) No. 7-287122 (JP7-287122A) and Japanese Laid-open publication (kokai) No. 7-287123 (JP7-287123A).
The optical films disclosed in these applications display superior optical characteristics including a high degree of transparency, a low retardation of transmitted light, and a uniform and stable application of retardation to transmitted light upon stretching and orientation, offer good levels of heat resistance and adhesion and bonding with other materials, and moreover also undergo little deformation on water absorption, and are consequently being investigated for potential application within a variety of optical film fields, as films capable of resolving the problems associated with optical films formed of conventional resins.
However, when optical films formed of conventional cyclic polyolefin-based resins are stretched and oriented, a function is obtained wherein retardation is imparted to transmitted light, although this function has a characteristic that the absolute value of the transmitted light retardation decreases as the wavelength of the transmitted light lengthens (hereafter this characteristic is described as a “negative wavelength dependency”). As a result, a retardation of, for example, ¼λ could not be imparted to transmitted light across the entire so-called visible light spectrum from 400 to 800 nm, using a single film. This property is true not only for optical films formed of conventional cyclic polyolefin-based resins, but also for optical films formed of the other resins described above.
As a result, conventionally, in order to impart a specified retardation, for example a retardation of ¼λ, to transmitted light within the visible light spectrum, a plurality of films for imparting retardation to the transmitted light (hereafter referred to as “retardation films”) needed to be laminated together (for an example refer to Japanese Publication of Patent No. 3174367). However, this method has many associated problems including the fact that precise lamination and angle adjustment is required during the lamination, the fact that some defective products are produced during the lamination, and the fact that the lamination increases the thickness of the retardation film. Accordingly, an optical film capable of imparting a specified retardation, for example a retardation of ¼λ, to transmitted light across the entire visible spectrum with a single sheet of film has long been sought.
In order to resolve this problem, an optical film for which the absolute value of the transmitted light retardation increases as the wavelength of the light lengthens, in other words an optical film which displays a positive wavelength dependency, is required. A retardation film formed of a specific cellulose acetate-based resin has been proposed as an optical film with this type of positive wavelength dependency (Japanese Laid-open publication (kokai) No. 2000-137116 (JP2000-137116A)). However, because this retardation film is formed of a cellulose-based resin, it suffers from problems such as variations in characteristics on water absorption and inferior heat resistance.